The present invention relates to lithium secondary batteries.
Currently, decreasing carbon dioxide, suppression of energy consumption, and the like are strongly required in view of environmental requirement. Accordingly, electric power storage systems, electric vehicles, and the like are receiving attention as new environmental technology. Lithium secondary batteries using non-aqueous electrolyte have been developed remarkably, because of their high battery voltage and high energy density, and the lithium secondary batteries are practically used for information apparatus such as computers, portable telephones, and the like.
However, because industrial batteries of high input, high output, and a large capacity require a large amount of active material, Co group materials and Ni group materials, which have been used for the information apparatus, can not be used practically for the industrial batteries in view of cost and resources. Therefore, spinel type Mn group materials are expected to solve these problems. However, the spinel type Mn group materials had problems such as low cycle life at high temperature, which is the most important issue for the industrial batteries, undesirable output characteristics, and undesirable input characteristics.
In order to apply the lithium secondary battery as power sources for electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like, a life at least 1000 cycles (at least 70% of capacity maintaining rate) at a high temperature higher than 50xc2x0 C., and an output power at least 500 W/kg are required for the lithium secondary batteries. However, conventional Mn group materials could not achieve such a long life nor high output power density.
Hitherto, many trials for extending the life have been performed. For instance, in accordance with JP-A-6-187993 (1994), extension of the life by increasing a composition ratio of Li and Mn, i.e. Li/Mn ratio, has been tried. However, decreasing its capacity of approximately several per cent was occurred after only 10 cycles of charge-discharge cycle even at room temperature. The cycle life of the lithium secondary battery is significantly influenced by environmental temperature, and, in particular, the life is remarkably shortened at a high temperature higher than 50xc2x0 C. Accordingly, it is difficult to obtain the cycle life longer than 1000 cycles at a high temperature higher than 50xc2x0 C. by only increasing the Li/Mn ratio.
In accordance with JP-B-8-24043 (1996), the extension of life has been tried by increasing the Li/Mn ratio similarly, and calcining the material at a temperature in the range of 430-510xc2x0 C. so as to obtain a material having a lattice constant smaller than 8.22 xc3x85. However, only a life of approximately 200 cycles at room temperature could be obtained, and any prospect to obtain a cycle life of at least 1000 cycles at a high temperature higher than 50xc2x0 C. could not be obtained. In accordance with JP-A-7-282798 (1995), the extension of life has been tried by using a material having a large Li/Mn ratio, i.e. Li[Mn2xe2x88x92xLix]O4 (0.020xe2x89xa6xxe2x89xa60.081). However, in a case when x was made 0.081 (Li/Mn ratio=0.58), decreasing the capacity of 5% was observed even in room temperature after approximately 100 cycles, and the cycle life longer than 1000 cycles could not be expected at a high temperature higher than 50xc2x0 C.
The reason of short cycle life is in disintegration of crystals of the positive electrode active material by repeating expansion and shrinkage of the positive electrode active material with plural cycles of charging and discharging operations, which makes it prohibit reversal absorption and desorption of lithium. Additionally, Mn ions are readily dissolved into the electrolyte at a high temperature, and the crystal of the positive electrode active material is more readily disintegrated than at room temperature. The dissolved ions are precipitated on the negative electrode, the charge and discharge reactions at the negative electrode is disturbed, and the life of the negative electrode is shortened.
The reason of low output characteristics, and low input characteristics with the lithium secondary battery is in low diffusion velocity relating to intercalation and deintercalation of lithium ions, because an organic solvent having a lower ion conductivity than aqueous solution is used as the electrolyte. In particular, a coating film is generated at the surface of the negative electrode by a reaction of the lithium ions, which are isolated by the presence of excessive amount of lithium ions for the reaction at the surface of the negative electrode, with the organic electrolyte, and the diffusion velocity of the lithium ions are further decreased by the presence of the coating film to make the output characteristics and the input characteristics worse. The ion conductivity of the organic solvent, i.e. the electrolyte, is decreased significantly at a low temperature, and the output characteristics and the input characteristics are getting further worse.
One of the objects of the present invention is to solve the above problems, and to provide a lithium secondary battery having a long life by using materials of long life at a high temperature, which is capable of receiving or supplying electric power rapidly corresponding to variation in power sources and power demands.
In accordance with the lithium secondary battery of the present invention, a material containing amorphous carbon is used for the negative electrode, and a complex oxide containing Li and Mn, which has a spinel type crystalline structure, is used for the positive electrode.
The complex oxide as the positive electrode of the present invention necessitates Li and Mn as indispensable elements, but a small amount of other elements such as transient element other than Mn, and elements in IIa group and IIIb group can be contained. For instance, these elements are Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Be, Mg, Ca, Sr, Ba, Ra, B, Al, Ga, In, Ti, and the like. Hereinafter structures of the positive electrode and the negative electrode are explained in details.
The positive electrode active material of the present invention is characterized in Li/Mn atomic ratio of the complex oxide in the range of larger than 0.55 and smaller than 0.80. When the Li/Mn atomic ratio is equal to or smaller than 0.55, the cycle life is short, because Mn ions are dissolved into the electrolyte and the crystalline structure of the complex oxide is disintegrated by repeating the charge and discharge cycles at a temperature higher than 50xc2x0 C. When the Li/Mn atomic ratio is equal to or larger than 0.80, the discharging capacity is small, and an objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The spinel type crystalline of the complex oxide of the present invention is characterized in its lattice constant in the range larger than 8.031 xc3x85 and smaller than 8.230 xc3x85. If the lattice constant is larger than 8.230 xc3x85, the cycle life is short, because Mn ions are dissolved into the electrolyte and the crystalline structure of the complex oxide is disintegrated by repeating the charge and discharge cycles at a temperature higher than 50xc2x0 C. If the lattice constant is smaller than 8.031 xc3x85, the discharging capacity is small, and an objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The crystalline of the complex oxide of the present invention is further characterized in its half value width of 2xcex8 angle at (400) peak in the x-ray diffraction pattern in the range smaller than 0.20xc2x0. In the measurement of the x-ray diffraction pattern, the Cu-kxcex1 ray was used as the radiation source, and a slit having a slit width of DS=SS=0.5, RS=0.15 was used. If the half value width is equal to or larger than 0.20xc2x0, the cycle life is short, because Mn ions are dissolved into the electrolyte and the crystalline structure of the complex oxide is disintegrated by repeating the charge and discharge cycles at a temperature higher than 50xc2x0 C.
The complex oxide of the present invention is further characterized in its specific surface of secondary particles of the complex oxide in the range smaller than 1.5 m2/g and larger than 0.10 m2/g. If the specific surface is larger than 1.5 m2/g, the cycle life is short, because Mn ions are dissolved into the electrolyte and the crystalline structure of the complex oxide is disintegrated by repeating the charge and discharge cycles at a temperature higher than 50xc2x0 C. If the specific surface is smaller than 0.10 m2/g, the power efficiency in quick charging and quick discharging is low, because the reaction field of the electrode active material itself is small, and an objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The complex oxide of the present invention is further characterized in its average particle diameter of primary particles of the complex oxide in the range larger than 1 xcexcm and smaller than 20 xcexcm. If the average particle diameter is smaller than 1 xcexcm, the cycle life is short, because Mn ions are dissolved into the electrolyte and the crystalline structure of the complex oxide is disintegrated by repeating the charge and discharge cycles at a temperature higher than 50xc2x0 C. If the average particle diameter is larger than 20 xcexcm, the power efficiency in quick charging and quick discharging is low, because the reaction field of the electrode active material itself is small, and an objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The positive electrode of the present invention can be used for obtaining the objective lithium secondary battery; which has a long life at a high temperature, and can be mounted on power sources of electric vehicles, electric power storage systems, elevators, electric tools, and the like; by only forming a combination with a negative electrode containing amorphous carbon. The negative electrode active material of the lithium secondary battery of the present invention is characterized in containing amorphous carbon, and in its negative electrode density in the range larger than 0.95 g/cm3, and smaller than 1.5 g/cm3.
When the charge and discharge cycles is repeated at a temperature higher than 50xc2x0 C., Mn ions are dissolved into the electrolyte from the positive electrode active material, and Mn is precipitated at a portion, the potential of which becomes lower than the Mn ion precipitation starting potential, i.e. 2 V. The precipitated portions are such as the negative electrode, separator, electricity collecting foil, battery can, and the like. If the negative electrode density is smaller than 0.95 g/cm3, vacancies in the negative electrode are numerous and the specific surface area as the electrode is large. Accordingly, a large amount of Mn is precipitated on the surface and inside of the negative electrode. The precipitated Mn decreases the capacity of the negative electrode significantly, and makes the cycle life short. If the negative electrode density is larger than 1.5 g/cm3, the vacancy in the negative electrode is too small to make the electrolyte penetrate into the inside of the electrode. Therefore, the capacity of the negative electrode is decreased significantly, and the objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The negative electrode active material of the lithium secondary battery of the present invention is further characterized in containing amorphous carbon, and in its true density in the range of 1.2 g/cm3-1.8 g/cm3. When the charge and discharge cycles is repeated at a temperature higher than 50xc2x0 C., Mn ions are dissolved into the electrolyte from the positive electrode active material, and Mn is precipitated at a portion, the potential of which becomes lower than the Mn ion precipitation starting potential, i.e. 2 V. The precipitated portions are such as the negative electrode, separator, electricity collecting foil, battery can, and the like. If the true density of the amorphous carbon is smaller than 1.2 g/cm3, vacancies in the carbon are numerous and the specific surface area is large. Accordingly, a large amount of Mn is precipitated on the surface and inside of the carbon. The precipitated Mn decreases the capacity of the negative electrode significantly, and makes the cycle life short. If the true density of the amorphous carbon is larger than 1.8 g/cm3, the vacancy in the negative electrode is too small to make the electrolyte penetrate into the inside of the electrode. Therefore, the capacity of the negative electrode is decreased significantly, and the objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The negative electrode active material of the lithium secondary battery of the present invention is further characterized in containing amorphous carbon and in its crystalline thickness Lc in the range of 5 xc3x85-150 xc3x85. The crystalline thickness Lc is one of the indexes indicating crystallinity of the carbon, that is, a small Lc indicates a strong amorphous property, and a large Lc indicates a strong graphitized property. The Lc is also an index indicating the number of laminated layers in a direction perpendicular to the six members ring network plane. A small Lc means a small number of the laminated layers, and further means a small number of intercalatingxe2x80x94deintercalating sites for lithium, i.e. the end portions of the six members ring. On the contrary, a large Lc means a large number of the laminated layers, and further means a large number of intercalatingxe2x80x94deintercalating sites for lithium, i.e. the end portions of the six members ring. If crystalline thickness Lc of carbon is smaller than 5 xc3x85, the intercalatingxe2x80x94deintercalating reaction is not proceeded smoothly, because the intercalating deintercalating sites for lithium is not ensured. Therefore, output characteristics and input characteristics are significantly deteriorated, because a condition wherein the lithium ions are strongly trapped in carbon is maintained. If crystalline thickness Lc of carbon is larger than 150 xc3x85, the properties as graphite becomes stronger than the amorphous properties, the six members ring network planes are laminated in parallel each other, and the end portion of the six members ring is concentrated in a direction. Therefore, the intercalatingxe2x80x94deintercalating sites for lithium is oriented in the direction, and the intercalatingxe2x80x94deintercalating reaction of lithium is proceeded only in a direction. Accordingly, the output characteristics and the input characteristics are significantly deteriorated. As the results, the objective lithium secondary battery for mounting on power sources of electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like can not be obtained.
The lithium secondary battery of the present invention is featured in obtaining an input density in the range of 300 W/kg-1800 W/kg per unit battery. Furthermore, the lithium secondary battery of the present invention is featured in obtaining an output density in the range of 500 W/kg-3500 W/kg per unit battery. The battery can be used in the same range as the output density. The power source which can be used in the above range is the power sources for such as electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like.
The lithium secondary battery of the present invention is featured in obtaining an input density in the range of 200 W/kg-1300 W/kg per set of batteries. Furthermore, the lithium secondary battery of the present invention is featured in obtaining an output density in the range of 360 W/kg-2520 W/kg per set of batteries. The battery can be used in the same range as the output density. The power source which can be used in the above range is the power sources for such as electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like.
The lithium secondary battery of the present invention is featured in obtaining an input density in the range of 300 W/kg-1800 W/kg per unit battery, and 200 W/kg-1300 W/kg per set of batteries, at a temperature in the range from xe2x88x9210xc2x0 C. to 50xc2x0 C. The battery can be used in the same range as the above. The power source which can be used in the above range is the power sources for such as electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like.
The lithium secondary battery of the present invention is featured in obtaining an input density in the range of 500 W/kg-3500 W/kg per unit battery, and 360 W/kg-2520 W/kg per set of batteries, at a temperature in the range from xe2x88x9210xc2x0 C. to 50xc2x0 C. The battery can be used in the same range as the above. The power source which can be used in the above range is the power sources for such as electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like.
A method of manufacturing the positive electrode active material of the present invention is featured in comprising the steps of: mixing manganese dioxide with lithium carbonate by a designated ratio, calcining preliminarily the mixture at a temperature in the range of 500xc2x0 C.-650xc2x0 C. in air, calcining again the mixture at a temperature in the range of 800xc2x0 C.-850xc2x0 C. in air for more than 20 hours, and cooling the mixture by a cooling velocity slower than 2xc2x0 C./minute. The positive electrode active material obtained by the method explained above has a high crystallinity, a significant grain growth, and preferable long life characteristics even at a high temperature.
The lithium secondary battery comprising the positive electrode of the present invention and the negative electrode has a cycle life of more than 1000 cycles at a temperature higher than 50xc2x0 C., and high input characteristics and output characteristics at a temperature in the range from xe2x88x9210xc2x0 C. to 50xc2x0 C. Therefore, the lithium secondary battery comprising the positive electrode of the present invention and the negative electrode can be applied to the power sources for such as electric vehicles, parallel hybrid electric vehicles, electric power storage systems, elevators, electric tools, and the like, in particular, to the power sources which requires power assistance.
In accordance with the present invention, the lithium secondary battery applicable to industrial batteries, which require a cycle life of more than 1000 cycles at a temperature higher than 50xc2x0 C., can be obtained.
Operations of the lithium secondary battery is explained, hereinafter.
In order to extend the chargingxe2x80x94discharging cycle life at a high temperature, it is necessary to suppress disintegration of crystalline structure accompanied with the charging-discharging reaction by increasing the stability of the positive electrode active material. The disintegration of crystalline structure accompanied with the charging-discharging reaction has two factors, the one is a mechanical disintegration caused by expansion and shrinkage of the lattice at the charging and discharging operation, and the other is a chemical disintegration caused by eluting tetravalent Mn generated during the charging operation by forming an organic complex with the organic solvent in the electrolyte.
In accordance with the positive electrode active material, a material having a large Li/Mn ratio is used. Accordingly, the ratio of Mn4+ ions having a smaller ion diameter in comparison with Mn3+ ions is increased relatively, and lattice deformation can be decreased by suppressing a Jahn-Teller unstability of Mn3+ ions. Therefore, both the mechanical disintegration and the chemical disintegration can be suppressed. For instance, when Li/Mn=0.50, in accordance with the chemical formula of LiMn2O4, an average valence of Mn ion is 3.5 in consideration of the neutral condition of its electric charge, that is, the number of Mn3+ ions is equal to the number of Mn4+ ions. When Li/Mn=0.58, an average valence of Mn ion calculated from the chemical formula of Li1+xMn2xe2x88x92xO4 becomes+3.63. It means that the ratio of Mn4+ ions is increased relatively.
At this time, the lattice constant is smaller than the lattice constant of the former. Accordingly, the amount of expansion and shrinkage during the charging and discharging is decreased, and the mechanical disintegration can be suppressed. If the valence of Mn comes to close to tetravalent, lithium ions which can not be deintercalated as much remain in the crystalline structure, and support the crystalline structure as operating as supporting poles. Accordingly, both the mechanical disintegration and the chemical disintegration can be suppressed.
Because the positive electrode active material of the present invention has a high crystallinity and significant grain growth, the stability of the crystal is remarkable, and both the mechanical disintegration and the chemical disintegration can be suppressed.
However, even if the positive electrode active material of the present invention is used, the elution of Mn to a certain extent can not be avoided depending on the temperature condition of the charging and discharging operations, although it may not cause the chemical disintegration. The problem caused by elution of Mn is the portion where the eluted Mn is precipitated. If the eluted Mn is precipitated on the negative electrode primarily, the negative electrode capacity is decreased, and the cycle life is shortened. The portions of precipitating Mn on the negative electrode can be decreased by increasing the density of the negative electrode, or the true density of the carbon, and decreasing the capacity can be suppressed.
As the material of the negative electrode for forming the battery, the material including amorphous carbon as the negative electrode must be used, in order to obtain industrial batteries of long life. When the negative electrode, which does not include amorphous carbon therein, is used, the cycle life is short, and the negative electrode can not be applied to industrial batteries, which require a cycle life of at least 1000 cycles even at a temperature higher than 50xc2x0 C. When negative electrodes made of conventional carbon other than the amorphous carbon are used, the organic solvent used as the electrolyte is readily decomposed at a temperature higher than 50xc2x0 C. to form carbon dioxide gas, hydrocarbons, lithium alkoxides, and others. In accordance with the amorphous carbon, such decomposition of the electrolyte is relatively less in comparison with other carbon materials, and the life at the high temperature is long.
As the carbon material for forming the battery, the carbon material having a crystalline thickness Lc in an optimum range must be used, in order to improve the output characteristics and the output characteristics. If the Lc is too large, or too small, it has undesirable effects such as decreasing the number of the intercalation-deintercalation sites, generating a directivity, decreasing the intercalating and deintercalating velocities, and others.
In accordance with the lithium secondary battery of the present invention, high input characteristics and output characteristics can be obtained by combining the positive electrode with the negative electrode of the present invention. Furthermore, in accordance with the lithium secondary battery of the present invention, high input characteristics and output characteristics can be obtained even if a set of the batteries is made up with it.